Valves are commonly used to control the flow characteristics of a fluid. A conventional valve includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A throttling element, such as a plug, is disposed in the path to control fluid flow. A stem connects the valve plug to an actuator, which moves the valve plug into and out of the flow path. A conventional valve may include a cage extending across the fluid flow path and having orifices formed therein. The plug has an outer surface sized to closely fit an inner wall of the cage. As a result, the plug is slidable inside the cage between a closed position, in which the plug obstructs the orifices, and an open position, in which the plug is clear of at least a portion of the orifices.
A seal is typically provided between the plug and the cage to prevent fluid from leaking therebetween. It is desirable to use a seal formed of a flexible material to ensure that a complete seal is formed between the plug and cage. It is common, for example, to provide a spring-loaded seal having a TEFLON® jacket. While the flexibility of materials such as TEFLON® improves the quality of the seal, such materials typically have a low hardness and therefore are susceptible to damage from foreign material in the vicinity of the seal. As a result, the seal is a high-wear component that requires routine replacement. While the frequency of replacement may vary greatly depending on service conditions, the seals will often last approximately 6-24 months in normal service conditions. In dirty service conditions, replacement of the seal may be more frequent. During seal replacement, the valve must be taken out of service and at least partially disassembled. As a result, such frequent seal replacement interrupts service, and is overly costly and time-consuming.
The spring-loaded seal having a TEFLON® jacket is useable in processes having a temperature of up to 450 degrees F. When the process temperature is above 450 degrees F., a risk arises that the seal may be extruded through the gap between the plug and cage. To prevent this problem, it is known to provide an anti-extrusion ring on one side of the seal. The anti-extrusion ring is typically attached to the plug and has an outer surface sized to slidably engage the inner surface of the cage, thereby preventing extrusion of the seal between the plug and cage. The anti-extrusion ring is typically formed of a high temperature thermal plastic, such as PEEK (polyetheretherketone), which retains its mechanical properties, such as flexural and tensile strength, at temperatures in excess of 450 degrees F. while being sufficiently soft to be non-damaging to the inner surface of the cage. While the use of an anti-extrusion ring allows the seal to be used in applications where the process temperature is above 450 degrees F., the use of a single ring has had no effect on the rate of wear of the seal.
In view of the foregoing, there is a need for a valve plug seal assembly having improved wear characteristics.